In the exploitation of undersea hydrocarbon reserves, it is frequently necessary to make a connection between a tubular member such as a riser pipe lowered from the surface and an existing riser base such as a well head.
One method for this is described in our earlier British Patent Application No. 7,928,006, now Pat. No. 2,056,009, which describes the use of a guide comprising a guide post with a hydraulic radially expandable end portion which is lowered through the tubular member, which is typically a riser pipe, on a cable with a hydraulic hose core to hang below the tubular member. A remote controlled vehicle (RCV) or diver then positions the guide so that its radially expandable portion is in the riser base where it is locked by hydraulic actuation of the radially expanding portion.
The riser pipe is then lowered, guided by the cable, to engage the guide post. As the pipe slips down over the guide post any angular misalignment is corrected by the guide post so that connecting members on the pipe and riser base are properly oriented for correction.
The guide previously described would typically weigh about 1000 lbs (454 Kg). The RCV's or divers employed to position this can move such a guide a small distance sideways as it hangs on its cable above the riser base but they cannot themselves lift the guide. It has now been appreciated that this may be generally somewhat disadvantageous but is especially so when the structure from which the tubular member and the guide are lowered is not itself fixed to the sea bed but are floating, for instance where the structure is a barge, ship or floating platform. Furthermore, there is a limit to the extent of sideways movement that can be achieved by an RCV or diver acting against the hanging weight of a guide as described above.
Large lateral offsets between the tubular member and riser base can be avoided using a fixed structure but are likely to be encountered in exploiting the proposed tethered buoyant platforms.
The first tethered buoyant platform (TBP) design contract was awarded in December 1979 and the oil industry will be able to evaluate this concept in prototype form by the mid 1980's. Floating production systems began with a semi-submersible rig having catenary mooring at Argyll field which came on stream in 1975. The advantages of the floating platform are their adaptability to deep water, and to marginal fields due to their mobility for re-use. These advantages have been known for at least five years but the excessive motions of the catenary moored system have delayed widespread application. Relative motions between riser and platform can be greatly reduced by the vertical tether system, see U.S. Pat. Nos. 3,780,685; 3,934,528, et al, and the engineering development of the first practical vertical tether system is under way.
Although vertical tethers reduce heave and pitch motions, they do not exercise the same restraint on lateral movement. Tether angles of 3.degree. in 500 ft. depth and 1.degree. in 2000 ft. depth will be fairly common. Even at 1/2.degree., the lateral offset in 500 ft. is 5 ft. and in 2000 ft. is 20 ft.
Drilling, production and sales risers must be run under these conditions, where a lateral offset of 20 feet between the top of a vertical riser and a seabed connection is likely.
Conventionally, drilling equipment is guided to the seabed by guide wires. Four wires are normally equally-spaced around a 12' diameter pitch circle. On a tension leg platform, the minimum number of conventional guide wires required would equal the number of well slots, but this quantity would only provide two wires per well. These wires are tensioned, and this total load would need to be considered as extra deck loading, thereby reducing the useful equipment capacity. Permanently installed wires will corrode and need periodic replacement, which could lead to entanglement.
An alternative solution to the lateral offset problem is suggested by U.K. Pat. No. 1,462,401 which describes a tethered buoyant platform with inclusive dynamic positioning means. Thrusters allow the platform
(a) to position itself directly above the subsea template and
(b) to guide risers into alignment with subsea connection points.
These thrusters will be used infrequently and are very expensive to install.
It is desired therefore to provide means first to guide the end of a tubular member such as a suspended riser to a position above the connection point, and then to guide the lower end of the riser to bring its axis into alignment with the sea bed connection means.